It clears your memory of unneeded files, hence reducing stuttering on PCs with less RAM and reducing or eliminating the infamous overfilled memory crash (game crashes because there is too much information stored in memory).

It dynamically changes various graphics options based on your framerate. If your PC is struggling, Streamline will lower your graphics settings and raise them when your PC can handle it.

It provides an overhauled autosave feature in the event that your game does crash. This system of saving has the same effect as using the save game function in the pause menu so it bypasses the quick and autosave function. This is the best autosave system I have seen so far.

There are 4 downloads provided on the Streamline link (namely v1, v2.1, v3.0 and v3.1).
I regard higher version numbers as being better but the modding community can't agree on which version is the most stable version of this mod. Most of the debate is between v3.0 and v3.1 with a small minority who stick with v2.1. I recommend you download and try every version to see which performs better on your system. If you don't feel like testing to find the best version, rather get v3.1 (the latest version). If you want more information on this mod, you will have to download it and read the readme.